DESCRIPTION: (Provided by the applicant) Breast cancer incidence is high in many developed countries and it is estimated that in the United States, 1 in 8 women will develop this disease. Although mortality from breast cancer has decreased in some age groups, it is still one of the leading causes of premature death in North American women. Research in this laboratory has identified selective Ah receptor modulators (SAhRMs) that are highly effective as inhibitors of mammary tumor growth in rodent models and are being developed for future clinical studies. We hypothesize that the mechanisms of inhibitory AhR-ERa crosstalk are dependent on cell- and gene promoter-context and involve both inhibitory dioxin responsive element (iDRE)-dependent and -independent pathways. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induces proteasome-dependent degradation of ERcx in breast cancer cells, and we hypothesize that this will result in limiting levels of cellular ERa and contribute to decreased expression of some genes. The molecular mechanisms of ligand [1 7B-estradiol (E2) and E2+TCDD]-induced proteasome degradation of ERa will be investigated in Aim 1. Aim 2 will investigate the role of decreased ERa levels and sequestration of ERa by the AhR as mechanisms for inhibitory AhR-ER crosstalk on genes containing functional estrogen responsive elements (EREs). Several E2-responsive genes in breast cancer cells are regulated by ERa/Sp1, and we hypothesize that the AhR complex may competitively displace ERa from binding Spi protein since these proteins bind to the same C-terminal domain of Spi. The mechanisms of AhR-mediated inhibition of genes with E2-responsive GC-rich promoters will be determined (Aim 3) using both functional and binding assays. Aim 4 will use wild-type, ERa and AhR knockout mice, and a transgenic mouse model expressing green fluorescent protein (GFP) regulated by the VEGF promoter to show that inhibitory AhR-ERa crosstalk on gene expression is also observed in vivo in the mouse uterus and mammary tumors. The proposed Aims will address important mechanistic considerations on IDRE-independent inhibitory AhR-ERa crosstalk and pathways critical for inhibition of mammary tumor growth by SAhRMs.